Sometimes the girls say: 'She is really good - I can't sing like that.' I explain that we can work together, and encourage them to use their voices."
Choral work features strongly at Sacred Heart, and the school has an accomplished gospel choir. "There are strong links between gospel and Ramp;B, which is hugely popular, particularly with girls," says Cath Roberts.
"Highlighting how Ramp;B grew out of gospel is a very good way of getting them interested in coming to choir.
"It has attracted a wider range of students than any other choir I have worked with. Years 7 and 8 often join a choir, but we also have a lot of students from Years 9, 10 and 11."
The girls suggest songs they feel might be suitable, although not everything can be made to work. "Some styles are difficult for group singing. But something from, say, Destiny's Child, where they sing in three-part harmony, can work."
When using contemporary material she is careful to venture only where she is sure of her own knowledge. "I pick songs that have a fairly classic approach, which I can link to songs that have gone before, to demonstrate progression. I would be much less confident going into recent developments in dance music - the latest garage or hip-hop, for example - because it changes so much, and I might say something that was completely farcical.
Students would think I didn't know what I was talking about, and they might even be insulted, because I was using their music wrongly.
"You also have to choose material that appeals to a broad base. Christina Aguilera is a good choice, because though some girls may not be fans of her style, they respect her for her singing."
The school has a music technology suite, which Cath Roberts says has made a major impact. "Even if students don't have the skills, the technology facilitates what they want to do."
Some pupils have been composing background music for film excerpts, using techniques gleaned from favourite TV programmes and movies. "When we listen to film music, I include the more classical composers, such as John Williams. It is often the more classical types of music that the students want to try to create," she says.